What are envelopes in my friends' towns for?

Envelopes are bonuses that you receive for helping your friends. Every time you help a friend, you earn reputation points. Once you collect enough of these points, you achieve new reputation level. The higher your reputation, the more you can help your friends. With each new level of reputation you will also receive greater rewards for visiting your friends, and your bonus for using your friends' help determined by their professions will increase as well. You can see your own profession by clicking on your name in the upper left corner of the game under the experience level.